Thin-film nanocomposite reverse osmosis membranes with enhanced antibacterial resistance by incorporating p-aminophenol-modified graphene oxide

Separation and Purification Technology 234 (2020) 116017

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Thin-film nanocomposite reverse osmosis membranes with enhanced antibacterial resistance by incorporating p-aminophenol-modified graphene oxide ⁎

T

⁎

Yan Zhanga,b,1, Huimin Ruana,1, Changmeng Guoa, Junbin Liaoa, , Jiangnan Shena, , Congjie Gaoa a b

Center for Membrane and Water Science & Technology, Zhejiang University of Technology, Hangzhou 310014, China Department of Chemical Engineering and Safety, Binzhou University, Binzhou 256603, China

A R T I C LE I N FO

A B S T R A C T

Keywords: p-Aminophenol-modified GO Thin film nanocomposite Antibacterial RO membrane Interfacial polymerization

Innovative approaches to restraint bacterial adhesion and growth on membranes are significantly needed to avoid membrane performance decaying for biofouling. In this work, a series of thin film nanocomposite (TFN) reverse osmosis (RO) membranes has been prepared by incorporating nano-fillers of p-aminophenol-modified graphene oxide (mGO) into the polyamide skin layer via interfacial polymerization. Our investigations demonstrate that the introduction of mGO nano-fillers into the functional skin layers reduces the hydrophilicities with the water contact angles drop from 69.6° to 48.2° and meanwhile decreases the thickness of functional skin layers from 240 nm to 50 nm, relative to pristine polyamide RO membrane without nano-fillers. As a result, the as-prepared TFN RO membrane at optimized conditions shows a water flux of 23.6 L·m−2·h−1 and a NaCl rejection rate of 99.7%, reflecting a remarkable promotion in water flux (by 24.5%) compared with the pristine RO membrane. In addition, the data statistics of the live/dead fluorescent imaging assay demonstrate that TFN RO membrane with mGO exhibits bacterial killing ratios of 96.78% and 95.26% against E. coli and S. aureus at the additive loading of 0.005 wt%, which are much higher than RO membrane with GO (90.64%; 90.43%) and pristine RO membrane (4.95%; 2.48%). This work demonstrates a facile way to TFN RO membranes with good separation performance and desirable antibacterial capacities.

1. Introduction Over the selective permeability process of a reverse osmosis (RO) membrane, accumulated solutes on the membrane surface during the RO operation can result in membrane fouling. The development of RO membranes has been greatly restricted by fouling, especially as a consequence of biofouling. Current methods of controlling membrane biofouling include: (i) sterilization using ozone, active chlorine and ultraviolet light in operation system; (ii) periodic chemical cleaning of RO membranes, and (iii) physicochemical modification of RO membranes. The first two methods have been widely used in RO systems, because it is the most practical and easiest solution for existing systems. However, because the aromatic polyamide layer is very sensitive to disinfectants and fungicides, the membrane surface is easily oxidized during the pretreatment and the membrane cleaning process, resulting in a degradation of the membrane structure. Therefore, new methods are required to control membrane fouling [1]. Recently, modification of

RO membranes with antibacterial nanomaterials has been considered as a promising method to increase the bacterial resistance of membranes [2,3]. The physicochemical modification of membrane surfaces can inhibit the adsorption of microorganisms on the membrane surface. Currently, the development of antibacterial RO membranes has been focused on the blending antibacterial nanomaterials into the casting solution, using the phase-inversion method or the interfacial polymerization process. Antibacterial nanomaterials for RO membranes suggest in literatures including chitosan [4], silver nanoparticles (AgNPs) [5], TiO2 nanoparticles [6,7], fullerene [8] and carbon nanotubes [9]. Special attention has been devoted to AgNPs as inorganic antibacterial nanoparticles. Lee et al. [10] embedded AgNPs in a polyamide (PA) composite RO membrane through arc plasma deposition to improve the membrane antibacterial properties. Over the process, AgNPs with the size of ~7.6 nm in diameter, were evenly distributed in the PA without particle agglomeration. The experimental results indicate that the

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Corresponding authors. E-mail addresses: [email protected] (J. Liao), [email protected] (J. Shen). 1 Contributed equally to this work and should be as the co-first authors. https://doi.org/10.1016/j.seppur.2019.116017 Received 18 April 2019; Received in revised form 21 July 2019; Accepted 2 September 2019 Available online 03 September 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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significant attention [36,37]. Many novel nanomaterials have served as nanofillers to enhance the permeability. For example, by introduction of GO in PA layer, the thickness can be finely tuned because of the hindrance effect and the confined GO channels allow fast passage of water molecules while preventing penetration of solutes [38,39]. Inspired by these, in this work, GO and modified graphene oxide by aminophenol (mGO) were used as nanofillers in the preparation of TFN composite RO membranes formed via interfacial polymerization. For mGO modified RO membrane, the use of mGO modified by aminophenol nanocomposites is a viable and attractive avenue for the development of anti-biofouling RO membranes.

addition of Ag in the RO membrane can improve the long-term antibacterial properties. Alternatively, TiO2 particles can be used; these are widely used as catalyst for organic matter degradation and bacterial inactivation by photocatalytic degradation. The dispersion of TiO2 nanoparticles into the aromatic thin film nanocomposite RO membrane structure was carried out through hydrogen bonds between nanoparticle and the eCOOH functional groups of aromatic polyamide [7,11]. Kim et al. [12] demonstrate that the introduction of TiO2 nanoparticles in a composite RO membrane by self-assembly in microbial destruction can achieve good antibacterial property. In recent years, graphene and graphene-based nanocomposites have been widely used in the preparation of antibacterial composite membranes [13]. Graphene has emerged as a remarkable material due to its unique properties of a high electrical conductivity, excellent mechanical strength and high surface area [14–16]. It has been reported that GO-based membranes have an enhanced water permeability, resulting from that GO is a typical 2D sheet-like nanomaterial with atomic thicknesses and can provides transport channels when properly stacked [17,18]. As a consequence, the development of GO membranes opened up a new field in water desalination, and in the permeation and vaporization of water/organic mixtures [19–21]. Pure GO membranes prepared by filtration are unstable and thus not suitable for water separation. In addition, in nanofiltration/reverse osmosis, a pure GO membrane could be easily damaged at high pressures and the GO would be peeled of its support. Therefore, to explore stable interactions between the GO nanosheet and appropriate polymers is necessary [22]. A proper mixing of atomic thickness GO into the polymer can significantly improve the physical properties of the polymer at very low concentrations [23–25]. In particular, the oxygen-containing functional groups in GO provide an excellent chemical stability, strong hydrophilicity and excellent antifouling properties for the preparation of functional nanocomposites, compared to a pure polymer. However, most reported studies have been devoted to the preparation of ultrafiltration [26–28] and pervaporation membranes [29,30], and only a few researchers have studied the effect of GO incorporation on RO membranes [31–33]. Perreault [34] proposed the use of GO particles for surface modification of polyamide composite membranes for the first time, to endorse antibacterial properties. The results indicate that around 65% bacteria were inactivated. Therein, the GO nanosheets were covalently bound to the polyamide active layer of the membrane, allowing an effective bacterial inactivation without altering the inherent water flux and desalination potential. The results demonstrate that GO has a good potential in designing new antibacterial composite polyamide membranes. For example, as per the report from Pant et al., phenols are capable of attacking both cell walls and membranes. This destructive attack has a significant impact on the permeability of cells, rendering the release of intracellular components (e.g., ribose, sodium glutamate) and interference with membrane function (electron transport, nutrient absorption, protein, nucleic acid synthesis, enzyme activity) [35]. Thus far, thin-film nanocomposite (TFN) membranes by incorporating the nanomaterials into PA layers have received

2. Experimental 2.1. Materials Polysulfone (PSf) ultrafiltration membrane was obtained from Hangzhou Water Development Center Co., Ltd. (China) and graphite powder was received from Sinopharm Chemical Reagent Co., Ltd. (China). p-Aminophenol, camphor sulfonic acid (CSA) and triethylamine (TEA) were received from Sigma-Aldrich and used as additives. 1,3-Phenylene diamine (MPD) and 1,3,5-benzenetricarbonyl trichloride (TMC) were obtained from Aladdin. Other chemicals, including potassium permanganate (KMnO4, 99%), sodium nitrate (NaNO3, 99%), sodium chloride (NaCl, 99%) and concentrated sulfuric acid (H2SO4, 98%) H2O2 (30 wt%) and hydrochloric acid (HCl, 37%) were purchased from Sigma-Aldrich. In addition, beef extract, peptone, yeast extract, agar and tryptone were received from Sinopharm Chemical Reagent Co., Ltd. Escherichia coli (E. coli) and Staphylococcus aureus (S. Aureus) were cultivated in our lab. 2.2. Preparation of GO and modified GO GO was prepared as per the modified Hummers method [40,41], of which the preparation procedure was described as follow (see the procedure in Fig. S1). 8.0 g graphite powder and 8.0 g NaNO3 were added slowly into 400 mL concentrated H2SO4, following by continuously stirring in an ice bath for 4 h. To the above mixture, 48.0 g KMnO4 was added slowly. After cooling down to room temperature, it was kept stirring for another 4 h. Before heating the mixture to 95 °C, 800 mL of DI water was added and then maintained it at 95 °C for 10 min. Subsequently, to the cooled mixture, 400 mL diluted H2O2 was added. The as-obtained mixture was washed and centrifuged repeatedly using 10% HCl and DI water until the pH was > 6, rendering the GO product after being fully freeze-dried. On the basis of as-prepared GO, herein, the preparation procedure of p-aminophenol modified GO (mGO) (see the chemical structure in Fig. 1) was described as follow. In detail, 1.0 g GO was dispersed in 200 mL THF under magnetic stirring for 24 h, followed with using ultrasonication for 1.5 h at 300 W. Meanwhile, 6.0 g of aminophenol was added in 400 mL ethanol and the resultant mixture was kept

Fig. 1. The chemical structure of p-aminophenol modified GO (mGO). 2

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addition, the schematic structure and experimental device have been also shown in Fig. S2 in Supplemental Information. The effective area of as-tested membrane is 25.5 cm2. Before testing, all membranes were pressurized at 1.5 MPa for 0.5 h to ensure the structure and performance stabilization in the following tests. Then, the filtration performance of all membranes was determined with a NaCl aqueous solution at 1.5 MPa, a temperature of 25 ± 2 °C and a pH of 7.0. The concentration of the NaCl aqueous solution was 2.0 g·L−1. The water flux (L·m−2·h−1) and the rejection (R, %) were calculated using Eq. (1) and Eq. (2), respectively. Each experiment was repeated for at least 3 times.

magnetically stirring for 12 h. Thereafter, the as-obtained two solutions were mixed under magnetic stirring for several hours and subsequently dispersed with ultra-sonication for another 2 h. At last, a brown solid was obtained (also see the procedure in Fig. S1 in Supplemental Information). 2.3. Preparation of RO membranes Two kinds of thin film nanocomposite (TFN) RO membranes were prepared by modifying a commercial PSf ultrafiltration (UF) membrane via interfacial polymerization [42] by incorporating the polyamide/GO and polyamide/mGO, respectively. Herein, we took the preparation of nanocomposite RO membranes with mGO for example and described the procedure as follow. A PSf UF membrane sheet in thickness of ca. 150 μm was immersed in a 50 mL water solution (aqueous phase) containing MPD/mGO [mGO additive loading (0.001–0.005 wt%)] with additives [TEA (2–3 wt%) and CSA (1–2 wt%)] for 2 min, following with removing the excess solution on the surface by nitrogen purging. Thereafter, the surface of membrane was exposed to 50 mL 0.1 wt% TMC in n-hexane solution (organic phase) for 30 s. Subsequently, the membrane was dried at 60 °C for 10 min and then immersed in a 2000 ppm Na2CO3 solution for 2 min. The obtained membrane was transferred to an oven for 5 min at 80 °C and soaked in DI water before testing. Regarding to GO modified RO membrane, its procedure is the same as that of mGO modified membranes. Table 1 lists the as-prepared nanocomposite RO membranes upon the additive loading and types of additives during the interfacial polymerization process. For the readability purpose, in this work, the four RO membranes with GO were denoted as G1, G2, G3 and G4, representing RO membranes prepared in 50 mL aqueous phase with the GO additive loading percentages of 0.001, 0.002, 0.003 and 0.005 wt%, respectively. In similar, M1, M2, M3 and M4 represent the four RO membranes prepared in 50 mL aqueous phase with the mGO with additive loading percentages of 0.001, 0.002, 0.003 and 0.005 wt%, respectively. In addition, the RO membrane without additive of GO or mGO but with polyamide, noted as P0, was made for comparison.

F=

V St

(1)

Cp ⎤ R (%) = ⎡1 − × 100 ⎢ Cf ⎥ ⎣ ⎦

(2)

where V is the volume (L) of permeated solution in a certain period of time t (h); S is the effective area (m2) of as-tested membrane; Cp is the concentration of permeate solution and Cf is concentration of the influent solution. 2.6. Evaluation of antibacterial performance Antibacterial tests for antibacterial performance evaluation of membranes were carried out by using two kinds of bacteria E. coli and S. aureus [44,45]. Bacteria were incubated at 37 °C for 24 h on agar plates. A single colony was taken out in 20 mL lysogeny broth (LB) and cultured at 200 rpm at 37 °C for 10 h. Concentration of bacteria was measured in term of optical density (OD) at 540 nm (OD = ~0.1). The membrane samples with the size of 1.5 cm × 1.5 cm, were firstly disinfected with 70% ethanol and PBS buffer. Subsequently, the samples were placed into 12-well plates with 2.0 mL of bacterium and incubated at 37 °C for 6 h. When the incubation was completed, the sample was rinsed with PBS buffer solution to remove excess bacteria that did not adhere to the membrane surface. Bacteria adhered to the membrane surface were stained by a Live/ Dead Back Light kit (Thermo Fisher Scientific Inc., NY); live bacteria were marked as green and dead bacteria as red. Fluorescence analysis method is used to quantitatively determine the antibacterial effect of bacteria. Photos of the bacteria were taken with a fluorescence microscope, and the antibacterial rate of each membrane sample was calculated by Eq. (3):

2.4. Membrane characterization An attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR, Nicolet 6700 spectrometer, US) was used to confirm the membrane surface structure. The surface and cross-sectional morphology of RO membranes were observed using scanning electron microscopy (SEM, HitachiS-4700(II)). The effect of GO and mGO additive loading on the membrane surface roughness was analyzed by atomic force microscopy (AFM, Agilent, 5500). The hydrophilicity of the membranes was evaluated by analyzing their water contact angle on Dataphysics (OCA30).

N K = ⎛1 − m ⎞ × 100% N0 ⎠ ⎝ ⎜

⎟

(3)

where K is the antibacterial rate, Nm is the number of live bacteria and N0 is the sum of dead and live bacteria. 3. Results and discussion

2.5. Separation performance tests

3.1. Surface characterization

The filtration performance of as-prepared membranes was tested using a cross-flow filtration membrane cell described elsewhere [43]. In

3.1.1. The surface structure Before confirming of as-prepared nanocomposite RO membranes with GO or mGO, the chemical structures of GO and p-aminophenol modified GO (mGO) were characterized by FT-IR. In Fig. 2(a), the spectra of GO shows the characteristic peaks of the stretching vibration of OeH (3430 cm−1), the stretching vibration of C]O from carboxyl group (1730 cm−1), the stretching vibration of C]C (1630 cm−1), the deformation vibration of OeH (1383 cm−1) and the CeO stretching vibration of epoxy (1230 and 1043 cm−1) [13,46]. The observable peaks are solid evidences for the present of eOH, eCOOH and CeOeC hydrophilic groups on GO. After modifying with p-aminophenol for mGO, the peak of carboxyl groups from GO disappears, but two newlyappeared peaks at 1650 and 1540 cm−1 were monitored on spectrum of

Table 1 The as-prepared RO membranes with different GO or mGO additive loadings (wt.%) via interfacial polymerization. RO Membranes

Additive loading

Modified with GO

Modified with mGO

wt.%

G1 G2 G3 G4

M1 M2 M3 M4

0.001 0.002 0.003 0.005

3

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Fig. 2. (a) ATR-FTIR spectra of GO and p-aminophenol modified GO (mGO); (b) Typical ATR-FTIR spectra of the RO membrane without GO or mGO (P0) and asprepared nanocomposite RO membranes with GO or mGO (G4 and M4).

50–80 nm, relative to those of G2–G4 ranging from 61 nm to 124 nm and that of P0 with 240 nm, when set with the same GO and mGO additive loading. It is long-known that interfacial polymerization is a diffusion-controlled chemical reaction, and the diffusion rate of the MPD and TMC is crucial for the structure and morphology of the modified layer [44,52]. However, the introduction of mGO fillers into the MPD and TMC decreased the diffusion rate of the MPD and TMC towards the organic phase. This is ascribed to the comprehensive effects of the steric hindrances of mGO fillers and the interactions between the MPD and TMC and the hydrophilic groups of the mGO fillers. In addition, the oxygencontaining functional groups in mGO can theoretically react with MPD, while the carboxyl or hydroxyl groups can interact with the acid chloride groups of the TMC structure, thus affecting the reaction rate between MPD and TMC. As a result, the interfacial polymerization process was suppressed, resulting in a thinner nanocomposite skin layer. With more GO and mGO additive loading, thinner skin layers are obtained. According to the work from Xia et al., too high concentration of mGO might result in the mGO agglomeration on the surface, thus negatively affecting the membrane structural integrity and thereby reducing the water flux [48]. Thus, herein the additive loading of GO and mGO is limited to 0.005%, this will be further confirmed by the results in the following discussions. Fig. 4 further shows the three-dimensional AFM surface morphologies of P0 and modified RO membranes of G3, G4, M3 and M4. Therein, all membranes have the similar “peak-valley” structure. We can see that the introduction of GO and mGO can render the decreased roughness of skin layers, which is in agreement with the SEM surface morphology. In detail, G3 and G4 have the roughness (Ra) of 72.2 nm and 68.6 nm, while M3 and M4 showing 67.4 nm and 56.5 nm, respectively. The reduction of skin layer roughness might be attributable to the delayed effect promoted by the GO nano-sheets on the diffusion of MPD into the organic solvent. Due to Langmuir-Blodgett membrane deposition, the GO nano-sheets on the PSf UF membrane may be arranged in the vertical direction when the GO/mGO-MPD solution is removed from the PSf support [53]. During the interfacial polymerization process, the diffusion of MPD leads to the formation of uplift. However, further MPD diffusion is restricted by the hindrance effect of the initially formed dense polyamide [54]. Similarly, the horizontal arrangement of GO also inhibits MPD diffusion, thus reducing both the roughness and the thickness of the PA layer [55]. As a result, the above surface morphology results possibly confirm that thin film nanocomposite RO membranes have been prepared.

mGO [46]. These two peaks should be assigned to the amide-I and amide-II band, respectively. The structures have been further confirmed by XPS spectra and Raman spectra and TEM images in Fig. S3 and Fig. S4, which are illustrated Supporting Information. The above results indicate that the amine groups of p-aminophenol have reacted with the carboxyl groups of GO and thus formed amide groups, suggestive of successful modification of GO. To clarify the chemical structure of GO or mGO modified RO membranes more, the highest GO or mGO additive loading RO membranes with enough characteristic groups should show the most observable characteristic peaks. Thus, herein, the FTIR spectra of as-prepared the nanocomposite RO membranes with the highest GO or mGO additive loading with the value of 0.005 wt%, corresponding to G4 and M4 have been typically and intentionally selected for FTIR characterization. Fig. 2(b) shows the FTIR spectra of G4 and M4, along with polyamide RO membrane without GO or mGO (P0) made for comparison. As reported, the most striking characteristic spectrum of polyamide is the band associated with the amide groups. Peaks at 1630 and 1540 cm−1 monitored on P0 are assigned to the amide-I and amide-II band, respectively [13,47,48]. With the introduction of GO and mGO into the MPD aqueous solution, some of the eOH and eCOOH groups on GO and mGO react with MPD to form amide bonds. Compared with P0, the GO modified PA membrane show absorption bands at ~3430 cm−1, ~1730 cm−1 and ~1650 cm−1, indicating that the present of GO with groups of eOeH, C]O and of C]C. In addition, the mGO-based membrane shows an amide group (e(C]O)NHe) peaks at 1650 cm−1 and 1540 cm−1, and a band at 1584 cm−1 assigned to the amide-I band from (e(C]O)NHe). Though the intensities of GO and mGO nano-filler peaks from FTIR spectra of G4 and M4 are reduced to some degree in Fig. 2(b), relative to those in Fig. 2(a), the incorporation of GO and mGO in the polyamide skin layer has been verified. 3.1.2. Surface morphology To investigate the effect of modification on surface structure of the as-prepared composite RO membranes, both SEM and AFM images have been captured. Herein, we have typically shown the SEM surface and cross-section images of P0 and composite RO membranes of GO (G2–G4) and mGO (M2–M4) in Fig. 3. From the surface images in Fig. 3(a), it can be seen that all RO membranes with and without GO and mGO show the valley-like structure. This kind of morphology agrees well with the results reported in many reports [49-51]. The leaflike structure is observed on surface of G2–G4 and M2–M4, compared with the nodular surface of P0. This is due to the anchored GO and mGO via the interfacial polymerization process. Accordingly, Fig. 3(b) shows their cross-sectional structure. We can see that the addition of GO and mGO significantly reduces the thickness of skin layers. In addition, M2–M4 show the thinner active layers with the thickness in range of

3.1.3. Water contact angle To further understand the surface property, the hydrophilicities of P0 and all nanocomposite RO membranes of G1–G4 and M1–M4 have 4

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Fig. 3. The surface (a) and cross-section (b) SEM morphologies of P0 and nanocomposite RO membranes of G2–G4 and M2–M4.

Fig. 4. The typical AFM morphologies of RO membranes without GO or mGO (P0) and modified RO membranes of G3, G4, M3 and M4. 5

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Fig. 5. Water contact angle of P0 and as-prepared nanocomposite RO membranes of G1–G4 and M1–M4.

0.005%. But to clearly illustrate the optimized composition conditions of TFN RO membranes, higher additive loadings of GO/mGO with 0.010% and 0.015% have been also included and their water fluxes and salt rejection rates have been shown in Fig. S5 in Supporting Information. Seen from the curves of G1–G4 in Fig. 6 and Fig. S5, with increasing additive loading of GO, both water flux and salt rejection rate increase gradually from 0.001% to 0.005%, and then decrease at additive loading 0.010% and 0.015%. The maximum water flux is 28.3 L·m−2·h−1 and its corresponding salt rejection rate is 99.9% at 0.005 wt%. This is due to that the water molecules can form hydrogen bonds with the oxygen atoms of the GO surface, and the water molecules undergo a friction-free slip under the action of hydrogen bonds [17]. With an increased GO additive loading, the hydrophilicity of RO membranes is increased, and the water channels constructed by GO also increases, thus the water flux increases. However, when the GO concentration is > 0.005 wt%, the polyamide layer shows varying degrees of defects. On the other hand, because the accumulated GO in the membrane surface exceeds the membrane surface thickness, the water molecules do not easily permeate through the membrane, so the water flux decreases. The water flux increases by 49.6%, compared with the pristine polyamide RO membrane (water flux 18.9 L·m−2·h−1, 99.5%) without adding GO/mGO. In addition, the increased GO additive loading has almost no influence on the rejection of a salt solution. As for TFN RO membranes with mGO, they show the similar water flux and salt rejection rate trend. The water flux and the salt rejection increase first and then decrease with increasing additive loading of mGO. The highest water flux is 23.6 L·m−2·h−1 and the salt rejection rate is 99.7% at mGO additive loading 0.003 wt%. Their slightly lower water fluxes can result from less hydrophilicity relative to RO membranes with GO, while smaller salt rejection rate might arise from thinner skin layer and reduced surface roughness as confirmed above. Even so, compared with pristine polyamide RO membrane, the maximum water flux of RO membranes with mGO is increased by 24.5%.

been evaluated by analyzing their water contact angle. Seen from Fig. 5(a), P0 without GO and mGO has the highest water contact angle of 69.8°, whereas the G1–G4 show the decreased values from 67.8° to 40.8°, with adding more amount of GO from 0.001 wt% to 0.005 wt%, indicative of the enhanced hydrophilicity of G1–G4. It is reasoned that the abundant hydrophilic groups (e.g., eOH and eCOOH) of GO dispersed on the surface of the modified RO membranes significantly contribute to the enhanced hydrophilicity. In similar, regarding to M1–M4, the incorporation of mGO also renders the decreased contact angles (Fig. 5(b)) with values from 69.6° to 48.2°. Compared to G1–G4, all nanocomposite RO membranes of M1–M4 show the slightly higher the water contact angles, respectively, which are in line with the change trend of surface morphologies by SEM and AFM as-discussed above. This is attributed to disappeared hydrophilic groups and denser skin layers (see the mGO of TEM morphology with increased number of lamellar nano-sheet layers in Fig. S4) on M1–M4 surface in interfacial polymerization process. The changed hydrophilicity and their sound difference possibly verify the preparation of thin film nanocomposite RO membranes. 3.2. Water flux and salt rejections To investigate the effects of the additive loading of GO and mGO on the filtration properties of as-prepared TFN RO membranes, the tests of water flux and salt rejection rate (NaCl) have been undertaken in Fig. 6. In consonance with the results from the discussion above, herein we still show the water flux and salt rejection rate of TFN RO membranes with additive loading of GO/mGO of 0.001%, 0.002%, 0.003% and

3.3. Antibacterial properties The antibacterial performances of TFN RO membranes G1–G4 and M1–M4 were investigated using E. coli and S. aureus as model bacterial strains. Fig. 7(a) and (b) show the live/dead fluorescence images of E. coli (a) and S. aureus (b) after exposure to pristine RO membrane (P0) and TFN RO membranes of G1–G4 and M1–M4 in shake cultivation for 6 h. Therein, the bacteria adhered to the membrane surface were stained, where live bacteria were marked as green and dead bacteria as red. Regarding to P0 samples, very few red spots appeared in the fluorescence images, indicating that almost no E. coli and S. aureus were sterilized. On the contrast, the density of distribution of red spots are gradually increased, with the increase of additive loading of GO and mGO. In particular, at 0.005 wt%, red spots are widely observed on G4

Fig. 6. Water flux (black) and salt rejection rate (red) vs. the additive loading of GO/mGO plot for RO membranes (G1–G4 and M1–M4) functionalized with GO (solid) and mGO (hollow). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 6

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Fig. 7. Live/dead fluorescence images of E. coli (a) and S. aureus (b) after exposure to RO membrane (P0) and TFN RO membranes of G1–G4 and M1–M4 in shake cultivation for 6 h.

functional groups on mGO, the oxidative stress reaction of bacteria and the hydroxyl groups on the aminophenol can play an antibacterial role [35]. In addition, mGO might also refer to the sharp edges contributing to the destruction of bacteria cells and mGO is an electron receptor to attract bacteria, resulting in bacterial disintegration and death [58].

and M4. Though it seems that RO membranes M1–M4 show more red spots relative to that G1–G4, both groups of experiments suggest the good antibacterial activity against both bacterial strains. To further quantitatively determine the antibacterial effect of bacteria, the data statistics of E. coli and S. aureus antibacterial performance of as-prepared RO membranes are summarized in Table 2, according to fluorescence microscope image by using Image-J software. We can see that the bacteria on P0 are mostly live, indicating that the P0 has no effect on bacteria growth, thus showing the bacterial killing ratios of E. coli and S. aureus for the P0 are only 4.9% and 2.5%. Both experiments on E. coli and S. aureus, the bacterial killing ratios is > 90%, when the GO or mGO additive loading is at 0.005 wt%. It is seen that a higher bacterial killing ratio from M1–M4 with mGO was observed against E. coli than against S. aureus, relative to the G1–G4 with GO because of the differences in the structures and chemical compositions of their cell walls. As reported, E. coli cells have thin layers of cross-linked linear peptidoglycans surrounded by an outer membrane comprising lipopolysaccharides, whereas S. aureus cells have thick cell walls composed of multiple layers of peptidoglycans and teichoic acid. The former has the thicker barriers than that in E. coli [39,56,57]. In this case with M1–M4 having bactericidal phenolic

4. Conclusions In this work, a series of TFN RO membranes has been prepared by incorporating nanofillers of p-aminophenol-modified GO into the polyamide skin layer on commercial PSf UF membrane via interfacial polymerization. The incorporation of mGO nanocomposites on membrane surface reduces the hydrophilicities and the thickness of functional skin layers, relative to pristine polyamide RO membrane. As a result, the as-prepared TFN RO membrane at optimized conditions shows a water flux of 23.6 L·m−2·h−1 and a NaCl rejection rate of 99.7%, reflecting a remarkable promotion in the water flux (by 24.5%) compared with the pristine RO membrane. In addition, RO membrane with mGO exhibits high bacterial killing ratios with the high values > 95% against E. coli and S. aureus. The investigations suggest that the use of mGO to prepare RO membranes can be an attractive avenue

Table 2 The data statistics of E. coli and S. aureus antibacterial performance of as-prepared RO membranes using fluorescence analysis method. Sample

Total number of bacteria

E. coli P0 G1 G2 G3 G4 M1 M2 M3 M4

323 299 318 301 299 301 295 305 311

± ± ± ± ± ± ± ± ±

3 2 3 1 2 2 1 1 2

Number of live bacteria

Bacterial killing ratio %

S. aureus

E. coli

S. aureus

E. coli

S. aureus

202 199 181 211 209 205 197 205 211

307 ± 4 211 ± 7 104 ± 3 47 ± 5 28 ± 2 142 ± 8 54 ± 3 14 ± 4 10 ± 2

197 ± 5 170 ± 3 134 ± 2 87 ± 4 20 ± 3 182 ± 5 99 ± 6 34 ± 2 10 ± 1

4.9 29.4 67.3 84.4 90.6 52.8 81.7 95.4 96.8

2.5 14.6 25.9 58.7 90.4 11.2 49.8 83.4 95.3

± ± ± ± ± ± ± ± ±

5 3 6 2 1 2 2 2 1

7

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to develop high-performance RO membranes with enhanced antibacterial resistance.

[22] N. Wang, S. Ji, J. Li, R. zhang, G. zhang, Poly(vinyl alcohol)-graphene oxide nanohybrid “pore-filling” membrane for pervaporation of toluene/n-heptane mixtures, J. Membr. Sci. 455 (2014) 113–120. [23] J. Lee, H.R. Chae, Y.J. Won, K. Lee, C.-H. Lee, H.H. Lee, I.C. Kim, J.M. Lee, Graphene oxide nanoplatelets composite membrane with hydrophilic and antifouling properties for wastewater treatment, J. Membr. Sci. 448 (2013) 223–230. [24] B.M. Ganesh, A.M. Isloor, A.F. Ismail, Enhanced hydrophilicity and salt rejection study of graphene oxide-polysulfone mixed membrane, Desalination 313 (2013) 199–207. [25] M. Hu, B. Mi, Enabling graphene oxide nanosheets as water separation membranes, Environ. Sci. Technol. 47 (2013) 3715–3723. [26] Z.W. Xu, J.G. Zhang, M.J. Shan, Y.L. Li, B.D. Li, J.R. Niu, B.M. Zhou, X.M. Qian, Organosilane–functionalized graphene oxide for enhanced antifouling and mechanical properties of polyvinylidene fluoride ultrafiltration membranes, J. Membr. Sci. 458 (2014) 1–13. [27] C. Zhao, X. Xu, J. Chen, Effect of graphene oxide concentration on the morphologies and antifouling properties of PVDF ultrafiltration membranes, J. Environ. Chem. Eng. 1 (2013) 349–354. [28] J. Zhang, Z. Xiong, X.S. Zhao, Graphene-metal-oxide composites for the degradation of dyes under visible light irradiation, J. Mater. Chem. 21 (2011) 3634–3640. [29] Y.Y. Lou, G.P. Liu, S.N. Liu, J. Shen, W.Q. Jin, A facile way to prepare ceramicsupported graphene oxide composite membrane via silane-graft modification, Appl. Surf. Sci. 307 (2014) 631–637. [30] Y.Y. Jiang, S.Q. Dai, X.Y. Yu, J.Z. Lin, S.X.X. Shi, T. Wang, L.G. Wu, Preparation and pervaporation property of GO-AgNP/PI mixed matrix membranes, Chin. J. Inorg. Chem. 32 (2016) 1345–1352. [31] Q. Liu, G.R. Xu, Graphene oxide (GO) as functional material in tailoring polyamide thin film composite (PA-TFC) reverse osmosis (RO) membranes, Desalination 394 (2016) 162–175. [32] J. Lee, J.H. Jang, H.R. Chae, S.H. Lee, C.H. Lee, P.K. Park, Y.J. Won, A facile route to enhance the water flux of a thin-film composite RO membrane: incorporating thickness-controlled graphene oxide into a highly porous support layer, J. Mater. Chem. A 3 (2015) 22053–22060. [33] H.M. Hegab, Y. Wimalasiri, M.G. Markovic, L. Zou, Improving the fouling resistance of brackish water membranes via surface modification with graphene oxide functionalized chitosa, Desalination 365 (2015) 99–107. [34] F. Perreault, M.E. Tousley, M. Elimelech, Thin-film composite polyamide membranes functionalized with biocidal graphene oxide nanosheets, Environ. Sci. Tech. Lett. 1 (2014) 71–76. [35] B. Pant, P. Pokharel, A.P. Tiwari, P.S. Saud, M. Park, Z.K. Ghouri, S. Choi, S.J. Par, H.Y. Kim, Characterization and antibacterial properties of aminophenol grafted and AgNPs decorated graphene nanocomposites, Ceram. Internat. 41 (2015) 5656–5662. [36] Y.L. Ji, Q.F. An, X.D. Weng, W.S. Hung, K.R. Lee, C.J. Gao, Microstructure and performance of zwitterionic polymeric nanoparticle/polyamide thin-film nanocomposite membranes for salts/organics separation, J. Membr. Sci. 548 (2018) 559–571. [37] D. Li, Y. Yan, H. Wang, Recent advances in polymer and polymer composite membranes for reverse and forward osmosis processes, Prog. Polym. Sci. 61 (2016) 104–155. [38] J. Yin, E.S. Kim, J. Yang, B. Deng, Fabrication of a novel thinfilm nanocomposite (TFN) membrane containing MCM-41 silica nanoparticles (NPs) for water purification, J. Membr. Sci. 423 (2012) 238–246. [39] L. Yu, W. Zhou, Y. Li, Q. Zhou, H. Xu, B. Gao, Z. Wang, Antibacterial thin-film nanocomposite membranes incorporated with graphene oxide quantum dot-mediated silver nanoparticles for reverse osmosis application, ACS Sustain. Chem. Eng. 7 (2019) 8724–8734. [40] X.L. Wang, H. Bai, G.Q. Shi, Size fractionation of graphene oxide sheets by pHassisted selective sedimentation, J. Am. Chem. Soc. 133 (2011) 6338–6342. [41] Y. Zhao, C. Zhou, J. Wang, H. Liu, Y. Xu, J.W. Seo, J. Shen, C. Gao, B. Van der Bruggen, Formation of morphologically confined nanospaces via self-assembly of graphene and nanospheres for selective separation of lithium, J. Mater. Chem. A 6 (2018) 18859–18865. [42] J.N. Shen, C.C. Yu, H.M. Ruan, C.J. Gao, B. Vander Bruggen, Preparation and characterization of thin-film nanocomposite membranes embedded with poly(methylmethacrylate)hydrophobic modified multiwalled carbon nanotubes by interfacial polymerization, J. Membr. Sci. 442 (2013) 19–26. [43] H.M. Ruan, C.M. Guo, H.W. Yu, J.N. Shen, C.J. Gao, A. Sotto, B.B. Van der, Fabrication of a MIL-53(Al) nanocomposite membrane and potential application in desalination of dye solutions, Ind. Eng. Chem. Res. 55 (2016) 12099–12110. [44] Y.F. Yang, H.Q. Hu, Y. Li, L.S. Wan, Z.K. Xu, Membrane surface with antibacterial property by grafting polycation, J. Membr. Sci. 376 (2011) 132–141. [45] L. Cen, K.G. Neoh, E.T. Kang, Surface functionalization technique for conferring antibacterial properties to polymeric and cellulosic surfaces, Langmuir 19 (2003) 10295–10303. [46] W. Shao, C. Liua, H. Ma, Z. Hong, Q. Xie, Y. Lu, Fabrication of pH-sensitive thin-film nanocomposite nanofiltration membranes with enhanced performance by incorporating amine functionalized graphene oxide, Appl. Surf. Sci. 487 (2019) 1209–1221. [47] A. Abrahamse, D.V. Chad, M. Elimelech, Covalent binding of single-walled carbon nanotubes to polyamide membranes for antibacterial surface properties, ACS Appl. Mater. Interfaces 3 (2011) 2869–2877. [48] Z.H. Wang, H.R. Yu, J.F. Xia, F.F. Zhang, F. Li, Y.Z. Xia, Y.H. Li, Novel GO-blended PVDF ultrafiltration membranes, Desalination 299 (2012) 50–54. [49] N. Ma, J. Wei, R.H. Liao, C.Y.Y. Tang, Zeolite-polyamide thin film nanocomposite membranes: Towards enhanced performance for forward osmosis, J. Membr. Sci.

Acknowledgement We express thanks for the funding support from the National Natural Science Foundation of China (No. 21676249), the National Natural Science Foundation of China (No. 21878273), the 63rd China Postdoctoral Science Foundation (No. 2018M632505) and the National Key Research and Development Plan (No. 2017YFC0403701). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.116017. References [1] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712–717. [2] Q.L. Li, S. Mahendra, D.Y. Lyon, L. Brunet, M.V. Liga, D. Li, P.J.J. Alvarez, Antibacterial nanomaterials for water disinfection and microbial control: potential applications and implications, Water Res. 42 (2008) 4591–4602. [3] D. Saeki, T. Tanimoto, H. Matsuyama, Anti-biofouling of polyamide RO membranes using phosphorylcholine polymer grafted by surface-initiated atom transfer radical polymerization, Desalination 350 (2014) 21–27. [4] L.F. Qi, Z.R. Xu, X. Jiang, C.H. Hu, X.F. Zou, Preparation and antibacterial activity of chitosan nanoparticles, Carbohydr. Res. 339 (2004) 2693–2700. [5] J.R. Morones, J.L. Elechiguerra, A. Camacho, K. Holt, J. Kouri, J.T. Ramírez, M.J. Yacaman, The bactericidal effect of silver nanoparticles, Nanotechnology 16 (2005) 2346–2353. [6] M. Cho, H. Chung, W. Choi, J. Yoon, Different inactivation behaviors of MS-2 phage and Escherichia coli in TiO2 photocatalytic disinfection, Appl. Environ. Microbiol. 71 (2005) 270–275. [7] S.Y. Kwak, S.H. Kim, Hybrid organic inorganic RO (RO) membrane for bactericidal anti-fouling. 1. Preparation and characterization of TiO2 nanoparticle self-assembled aromatic polyamide thin-film-composite (TFC) membrane, Environ. Sci. Technol. 35 (2001) 2388–2394. [8] D.Y. Lyon, L.K. Adams, J.C. Falkner, P.J.J. Alvarez, Antibacterial activity of fullerene water suspensions: effects of preparation method and particle size, Environ. Sci. Technol. 40 (2006) 4360–4366. [9] S. Kang, M. Pinault, L.D. Pfefferle, M. Elimelech, Single-walled carbon nanotubes exhibit strong antibacterial activity, Langmuir 23 (2007) 8670–8673. [10] S.H. Park, S.H. Kim, S.J. Park, S. Ryoo, K. Wooc, J.S. Lee, T.-S. Kim, H.-D. Park, H. Park, Y.-I. Park, J.H. Cho, J.-H. Lee, Direct incorporation of silver nanoparticles onto thin–film composite membranes via arc plasma deposition for enhanced antibacterial and permeation performance, J. Membr. Sci. 513 (2016) 226–235. [11] H.S. Lee, S.J. Im, J.H. Kim, H.J. Kim, J.P. Kim, B.R. Min, Polyamide thin-film nanofiltration membranes containing TiO2 nanoparticles, Desalination 219 (2008) 48–56. [12] S.H. Kim, S.Y. Kwak, B.H. Sohn, T.H. Park, Design of TiO2 nanoparticle self-assembled aromatic polyamide thin-film-composite (TFC) membrane as an approach to solve biofouling problem, J. Membr. Sci. 21 (2003) 157–165. [13] S. Bano, A. Mahmood, S.J. Kim, K.H. Lee, Graphene oxide modified polyamide nanofiltration membrane with improved flux and antifouling properties, J. Mater. Chem. A 3 (2015) 2065–2071. [14] D.R. Dreyer, S. Park, C.W. Bielawski, The chemistry of graphene oxide, Chem. Soc. Rev. 39 (2010) 228–240. [15] M. Koinuma, C. Ogata, Y. Kamei, K. Hatakeyama, H. Tateishi, Y. Watanabe, T. Taniguchi, K. Gezuhara, S. Hayami, A. Funatsu, M. Sakata, Y. Kuwahara, S. Kurihara, Y. Matsumoto, Photochemical engineering of graphene oxide nanosheets, J. Phys. Chem. C 116 (2012) 19822–19827. [16] D.A. Dikin, S. Stankovich, E.J. Zimney, R.D. Piner, G.H.B. Dommett, G. Evmenenko, S.T. Nguyen, R.S. Ruoff, Preparation and characterization of graphene oxide paper, Nature 448 (2007) 457–460. [17] R.R. Nair, H.A. Wu, P.N. Jayaram, I.V. Grigorieva, A.K. Geim, Unimpeded permeation of water through helium-leak-tight graphene-based membranes, Science 335 (2012) 442–444. [18] R.V. Gorbachev, J.C.W. Song, G.L. Yu, A.V. Kretinin, F. Withers, Y. Cao, A. Mishchenko, I.V. Grigorieva, K.S. Novoselov, L.S. Levitov, A.K. Geim, Detecting topological currents in graphene superlattices, Science 346 (2014) 448–451. [19] T.M. Yeh, Z. Wang, D. Mahajan, B.S. Hsiao, B. Chu, High flux ethanol dehydration using nanofibrous membranes containing graphene oxide barrier layers, J. Mater. Chem. A 1 (2013) 12998. [20] D.P. Suhas, A.V. Raghu, H.M. Jeong, T.M. Aminabhavi, Graphene-loaded sodium alginate nanocomposite membranes with enhanced isopropanol dehydration performance via a pervaporation technique, RSC Adv. 3 (2013) 17120. [21] N.X. Wang, S.L. Ji, G.J. Zhang, J. Li, L. Wang, Self-assembly of graphene oxide and polyelectrolyte complex nanohybrid membranes for nanofiltration and pervaporation, Chem. Eng. J. 213 (2012) 318–329.

8

Separation and Purification Technology 234 (2020) 116017

Y. Zhang, et al.

[54] R. Shimizu, T.E. Everhart, N.C. Macdonald, C.T. Hovland, Edge effect in high-resolution scanning Auger-electron microscopy, Appl. Phys. Lett. 33 (1978) 549–551. [55] H.R. Chae, J. Lee, C.H. Lee, I.C. Kim, P.K. Park, Graphene oxide-embedded thin-film composite ROmembrane with high flux, anti-biofouling, and chlorine resistance, J. Membr. Sci. 483 (2015) 128–135. [56] M. Caroff, D. Karibian, J.M. Cavaillon, N. Haeffner-Cavaillon, Structural and functional analyses of bacterial lipopolysaccharides, Microbes Infect. 4 (9) (2002) 915–926. [57] A. vander Wal, W. Norde, A.J.B. Zehnder, J. Lyklema, Determination of the total charge in the cell walls of Gram-positive bacteria, Colloids Surf. B 9 (1–2) (1997) 81–100. [58] J.H. Li, G. Wang, H.Q. Zhu, M. Zhang, X.H. Zheng, Z.F. Di, X.Y. Liu, X. Wang, Antibacterial activity of large-area monolayer graphene film manipulated by charge transfer, Sci. Rep. 4 (2014) 4359.

405–406 (2012) 149–157. [50] Y.H. Cho, J. Han, S. Han, M.D. Guiver, H.B. Park, Polyamide thin-film composite membranes based on carboxylated polysulfone microporous support membranes for forward osmosis, J. Membr. Sci. 445 (2013) 220–227. [51] G. Han, S. Zhang, X. Li, N. Widjojo, T.S. Chung, Thin film composite forward osmosis membranes based on polydopamine modified polysulfone matrixs with enhancements in both water flux and salt rejection, Chem. Eng. Sci. 80 (2012) 219–231. [52] S.j. Xia, L.J. Yao, Y. Zhao, N.N. Li, Y. Zheng, Preparation of graphene oxide modified polyamide thin film composite membranes with improved hydrophilicity for natural organic matter removal, Chem. Eng. J. 280 (2015) 720–727. [53] Y.J. Yan, X.J. Yang, W.Y. Yang, S.B. Li, J.H. Xu, Y.D. Jiang, Ordered and ultrathin reduced graphene oxide LB films as hole injection layers for organic light-emitting diode, Nanoscale Res. Lett. 9 (2014) 537-537.

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